Processes for manufacturing fibrous nonwoven sheets from polyolefin polymers are known in the art. Blades et al., U.S. Pat. No. 3,081,519 (assigned to E.I. DuPont de Nemours & Company (hereinafter "DuPont")), discloses flash-spinning of plexifilamentary polyethylene film-fibrils. Steuber, U.S. Pat. No. 3,169,899 (assigned to DuPont), discloses depositing a flash-spun polyethylene plexifilamentary film-fibril web onto a moving belt and compressing the deposited web to form a lightly consolidated nonwoven sheet. The term "plexifilamentary" means a three-dimensional integral network of a multitude of thin, ribbon-like, film-fibril elements of random length and with a mean thickness of less than about 4 microns, and with a median fibril width of less than about 25 microns. In plexifilamentary structures, the film-fibril elements are generally coextensively aligned with the longitudinal axis of the structure and they intermittently unite and separate at irregular intervals in various places throughout the length, width and thickness of the structure to form the three-dimensional network.
In order to produce sheets with the strength and barrier properties required for many applications, such as air infiltration barrier sheet material used in home construction (housewrap), the film-fibrils or other fibers of the lightly consolidated sheet material must be bonded together. Lightly consolidated nonwoven sheets made from polyolefin fibers have been bonded by calendering and hot air treatments. However, sheets so bonded have tended to melt, shrink and curl, resulting in sheets with irregular thickness, opacity, strength and permeability properties.
A process for bonding polyolefinic plexifilamentary, film-fibril sheets with properties sufficiently uniform for commercial applications is disclosed in David, U.S. Pat. No. 3,532,589 (assigned to DuPont) and is shown in FIG. 1. The thermal bonding process disclosed in the David patent requires that the unconsolidated film-fibril sheet 5 from supply roll 6 be subjected to light compression during heating in order to prevent shrinkage and curling of the bonding sheet. A flexible belt 2 is used to compress a sheet being bonded against a large heated drum 1 that is made of a heat-conducting material. Tension in the belt is maintained by the rolls 3. The belt is preheated by a heating roll 9 and a heated plate 10. The drum 1 is maintained at a temperature substantially equal to or greater than the upper limit of the melting range of the film-fibril elements of the sheet being bonded. The rotating heated drum 1 is large (about 2 m in diameter) so as to permit the film-fibril sheet to be heated long enough to allow the face of the sheet against the roll to reach a temperature within 7.degree. C. of the upper limit of the melting range of the film-fibril elements, but not substantially above said upper limit, and to allow the second face of the sheet to reach a temperature between 0.8.degree. to 10.degree. C. lower than the temperature of the first face of the sheet. The heated sheet 5 is removed from the heated drum 1 without removing the belt restraint and the sheet is then transferred to a cooling roll 4 where the temperature of the film-fibril sheet throughout its thickness is reduced to a temperature less than that at which the sheet will distort or shrink when unrestrained. Roll 7 removes the bonded sheet from the belt 2 before the sheet is collected on a collection roll 8. The sheet may be run through another thermal bonding device like that shown in FIG. 1 with the second surface facing the heated drum in order to produce a hard bonded surface on the opposite side of the sheet.
For the past twenty-five years, a thermal bonding process similar to that shown in FIG. 1 has been applied to the commercial production of hard-surfaced spunbonded polyolefin sheet material, such as TYVEK.RTM. spunbonded polyethylene sheet sold by DuPont. TYVEK.RTM. is a registered trademark of DuPont. This experience has demonstrated that the bonding apparatus shown in FIG. 1 is costly to construct, operate and maintain. The large heating drums are expensive to construct, they require large amounts of energy to heat, and their surfaces are difficult to keep clean. The flexible belt 2 used in the prior art process is similarly expensive to heat and maintain. In addition, the bonding process shown in FIG. 1 offers little flexibility for altering the degree of bonding in a sheet product or for producing sheet structures that are extra highly impermeable to air and water, while maintaining good moisture vapor transmissibility. Finally, the bonding process shown in FIG. 1 cannot be used to produce an embossed, point bonded, or otherwise patterned sheet without additional processing steps. Accordingly, there is a need for a lower cost process for bonding plexifilamentary film-fibril sheet material that also offers the flexibility to produce a variety of bonded sheet products including sheet structures that have excellent strength yet are also very smooth and printable, and sheet structures that are highly impermeable to air and water, but demonstrate good moisture vapor transmissibility.